Cold-reactive AIHA occurs when IgM binds to which polysaccharide antigen on RBCs?

Cold-reactive autoimmune hemolytic anemia (AIHA) is a condition where the immune system produces antibodies against red blood cells (RBCs) at lower temperatures, leading to hemolysis. In this type of AIHA, the predominant antibody involved is usually of the IgM class, which can bind to specific antigens present on the surface of RBCs.

The correct answer, which is the I antigen, plays a crucial role in this process. The I antigen is a carbohydrate structure found on the membranes of normoblasts and mature red blood cells. In patients with cold-reactive AIHA, IgM antibodies target these I antigens, particularly in colder environments. This binding initiates agglutination and complement activation, leading to the destruction of the RBCs, primarily in the liver and spleen.

The significance of the cold-reactive nature of the antibodies arises from the fact that their action is more pronounced or only occurs at reduced temperatures, such as when a person is exposed to cold weather or environments. Thus, in cold-reactive AIHA, the immunological response is driven specifically by the interaction between IgM antibodies and the I polysaccharide antigen on the RBC surface.